Under normal conditions, a healthy human eye focuses on near and distant objects by contraction and relaxation of the ciliary muscle thereby contracting and releasing the tension on the zonules in the eye. The elastic forces of the eye cause disaccommodation and the elastic recoil of the lens caused accommodation. The balance between these two opposing elastic forces is modulated by the neurologically controlled contraction of the ciliary body. The contraction of the ciliary muscle releases zonular tension (accommodative state) and allows the lens to return to a more globular or spherical resting shape. The relaxation of the ciliary muscle increases tension on zonules and elastic forces in the eye tissue overcome the inherent lens elasticity and result in stretching the lens equator and flattening the lens curvature (disaccommodative state).
In certain instances, for example when age-related opacification of the lens (cataract) interferes with vision, the natural crystalline lens of the eye needs to be removed. Generally, the natural lens is replaced with an artificial one, for example, an intraocular lens (IOL).
Unfortunately, conventional IOLs, and currently FDA approved accommodative IOLs, may be unable to provide sufficient spatial displacement of the lens along the optical axis to provide an adequate amount of accommodation for near vision. For an accommodative IOL to be effective, it preferably provides equally for both accommodation and disaccommodation.
In conventional extracapsular cataract surgery, the crystalline lens matrix is removed by phacoemulsification through a curvilinear capsularhexis leaving intact the thin walls of the anterior and posterior capsules, together with zonular ligament connections to the ciliary body and ciliary muscles. An intraocular lens is then placed in the capsular bag, which collapses around the IOL. A conventional monofocal IOL is rigidly fixated in the nonmoving and fibrosed capsular bag. The position of the IOL in the capsular bag is neither in the accommodated or disaccommodated state, but somewhere in between, as determined by the amount of bag contraction and IOL design. This position is called “effective lens position” and it is utilized in calculating the power of the desired optic. The power of the optic determines the single point of perfectly focused vision, often selected at a practical arm length range.
Conventional axial shifting accommodative intraocular lenses (AIOL) rely on the interaction of the ciliary muscle with the zonule and capsule and the vitreous fluid compartment to induce movement of the optic of the AIOL along its optical axis. Typically, the AIOL is secured within the capsular bag that attempts to translate both the rotational and the radial stretching force exerted by the zonules in an attempt to achieve the desired axial displacement of the optic.
However, during the post-implantation fibrotic healing process, the anterior capsule fuses with the posterior capsule to form a rigid capsular disc, a strait jacket for the AIOL. Loss of elasticity of the capsular disc results and constrains the amount of movement, both centrifugal and rotational, that can be generated by the eye disaccommodation forces transmitted to the IOL via zonules or by the elastic recoil of the intraocular lens within the bag and therefore, leads to a decrease in the amount of axial displacement of the lens that can be achieved. The lens neither accommodates nor disaccommodates.
Various lens systems have been designed to address this loss of accommodation. One type of passive-shift single-optic lens, the only accommodative lens currently marketed in the United States, was designed to move forward under vitreous humor pressure when presumably the ciliary muscle contracts and forces vitreous forward. Even the limited amount of accommodative amplitudes generated by this lens immediately after surgery may be lost within the first few weeks or month after surgery as capsular fibrosis ensues and sometimes the movement is backwards. No passive shift AIOLs are marketed in the US that translate ciliary muscle contraction into forward shift of the optic by direct mechanical action of the haptics.
Accommodative lens designs with single or multiple optic lens assemblies have been disclosed, for example, in U.S. Patent Publication Nos. 2009/0125106, 2005/0209692, 2007/0156236, 2009/0005866, 2007/0005136, and 2009/0248154. Dual optic lenses retain the problem of capsular fibrosis and loss of amplitude/movement even though they are theoretically expected to provide a significant amount of accommodation. One such IOL received regulatory approval in Europe, but was abandoned and withdrawn from the market because of poor and unpredictable clinical results. This lens did not receive FDA approval and currently this approach appears to have been abandoned.
More recently, a lens systems that employs an active-shift mechanism using repulsive mini-magnets as a means of making accommodation partially independent of the zonules and mechanical properties of the capsular bag was disclosed (see U.S. Patent Publication Nos. 2009/0204210 and 2007/0118216). Still other methods of achieving accommodation include introduction of a polymerizable fluid with a desired refractive index into the capsular bag (lens refilling). Extensive investigation into the feasibility of these methods is still needed.
U.S. Patent Publication No. 2009/0234449 discloses an intraocular lens having an accommodating element that is in contact with a substantial portion of the zonular contact region of the ciliary body; the accommodating element is positioned relative to optical element and configured to cooperate with the ciliary muscle, the zonules and/or the vitreous pressure in the eye to effect a shape change to the optical element. According to the '449 publication, prior art multiple lens systems can be cumbersome and may also require an axial displacement unachievable with a collapsed capsular bag and resulting ineffective accommodative mechanisms.
More recently, a lens system has been described that employs a novel zonular capture haptic (ZCH) (see U.S. Patent Publication No. 2011/0307058, incorporated herein by reference in its entirety). The lens system provides improved accommodation via a two stage procedure. In the implantation step, Stage 1, a specially designed sectionable haptic, i.e., a ZCH, is inserted between the anterior and posterior lens capsules and it is maintained in a restrained configuration. Sufficient time is allowed for fusion and fibrosis of the two capsule leaflets to each other, thereby permanently trapping the haptic components between the capsules. During Stage 2, activation surgery may be subsequently performed to section the fused capsular bag between individual haptic components and release the restraining mechanism, thereby breaking the mechanical restraint that typically limits movement of other “in the bag” implanted IOLs.